1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and to a process for its fabrication.
2. Background Art
Recent years have seen increasingly accelerated digitalization, miniaturization and higher speeds of electronic devices. Electrolytic capacitors are high frequency application-suited electronic parts which are abundantly employed in various types of electronic devices, and hence they are being required to exhibit even higher capacitance and lower impedance during high-frequency operation than in the past, while superior properties such as operational stability, reliability and extended usable life are also greatly desired.
Electrolytic capacitors are generally fabricated by providing a valve metal layer made of aluminum, tantalum or the like, the surface of which is successively laminated with a dielectric layer made of an oxide film formed by anodic oxidation, an electrolyte layer and a conductor layer made of graphite, silver or the like.
Such electrolytic capacitors are largely classified into two types: electrolytic solution capacitors and solid electrolytic capacitors, based on the properties of the electrolyte material. The former type possesses an electrolyte layer comprising a liquid electrolyte (electrolytic solution) as the electrolyte material, while the latter possesses an electrolyte layer comprising a solid electrolyte (complex salt, conductive polymer, etc.) as the electrolyte material. When compared in terms of their properties, the former is intrinsically prone to deterioration with time due to leakage or evaporation (dry-up) of the electrolyte, while the latter is essentially free of such risk.
Because of this advantage, research and development of solid electrolytic capacitors have been actively pursued in recent years, and particularly in consideration of the leakage current value, impedance characteristics and heat resistance, the focus of development and implementation has drastically shifted from capacitors employing manganese dioxide or complex salts to those employing conductive polymers prepared by doping conjugated polymer compounds such as polypyrroles, polythiophenes and polyanilines with electron donor or electron acceptor substances (dopants).
Incidentally, the valve metal layer of an electrolytic capacitor having the general construction described above is usually etched to achieve higher capacitance, and the surface thereof therefore has a fine irregular shape. Consequently, the dielectric layer formed thereover also exhibits the same fine irregular shape. The dielectric layer can suffer serious damage and even loss of function due to natural deterioration when the electrolytic capacitor stands for long periods without a circuit load, or due to sudden temperature changes, electrical shock (by application of overvoltage, reverse voltage or excess ripple current) or physical shock.
When such damage occurs, the electrolytic capacitor exhibits phenomena such as increased leakage current and subsequent short circuiting. It is therefore considered absolutely essential for an electrolytic capacitor to have the property of self-repairing the damaged portions of the dielectric (hereinafter referred to as “self-repair function”).
In an electrolytic capacitor employing the aforementioned electrolytic solution, the valve metal which is emergent (exposed) at the damaged sections is in contact with the electrolytic solution. The electrolytic solution contains ionic molecules or compounds, and application of a prescribed rated voltage to the electrolytic capacitor causes oxidation of the valve metal by oxygen produced from the ionic molecules or compounds, thereby regenerating the damaged sections of the dielectric.
In contrast, essentially no ionic migration occurs in a solid electrolytic capacitor. It is therefore difficult to achieve the repair function described above. Emergence of immediately local damaged sections results in formation of current pathways and local buildup of Joule heat by the generated current, which heat converts a portion of the solid electrolyte to a non-conductor and can block the current pathway. In the case of very extensive damage or large damaged regions, however, repair cannot occur and short circuiting results.
It has therefore been attempted to impart a self-repair function to conductive polymer-type solid electrolytic capacitors while maintaining their excellent properties and physical attributes. For example, Japanese Patent Laid-Open Publication No. HEI 11-283874 and Japanese Patent Laid-Open Publication No. 2000-21689 have proposed solid electrolytic capacitors employing both an electrolytic solution and a conductive polymer compound as electrolytes.